1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus, and more particularly, to a white reference member for correcting image information obtained by reading an original.
2. Description of the Related Art
Up to now, as an image forming apparatus such as a digital copier, a printer, or a facsimile machine, there is an image forming apparatus including an image forming unit and an image reading apparatus, for causing the image forming unit to form an image based on image information read by the image reading apparatus. As a reading method performed by the image reading apparatus, there is a method of reading an original image, in which an original, which being a sheet, is placed on a platen glass plate, the original is then brought into intimate contact with the platen glass plate by a pressure plate, and an image reading unit performs scanning in this state.
Further, as the reading method performed by the image reading apparatus, there is a method (hereinafter referred to as “flow reading”) of using the image reading unit to read the original image by stopping the image reading unit under a predetermined original reading position of the platen glass plate and conveying the original over the original reading position by using an ADF at a constant speed.
The image reading unit is provided with a light source being a lighting device for illuminating the original. As the light source, in recent years, there is a light source using an LED array obtained by aligning multiple LEDs (light emitting diodes) in a straight line for purposes of low power consumption, downsizing, and the like. In order to maintain image reading accuracy at a constant level, the image reading apparatus periodically executes shading correction for correcting unevenness in density level that occurs due to characteristics of an image pickup element, influence of a lens aberration, and the like. In a case where the flow reading is continuously performed by using the ADF, a shading correction value is acquired before reading the first original, and then the shading correction is performed by using the same shading correction value until the continuous flow reading is finished.
In the case where the flow reading is continuously performed, factors relating to image quality highly depend on the light source, and deterioration of the light source greatly influences the reading of the original. Specifically, after the light source starts illuminating, an illuminance and a distribution of light amounts in a main scanning direction fluctuate due to a change in temperature of the light source. Therefore, in a case of continuously reading originals, the influence of the deterioration of the light source appears between, for example, data obtained by reading the first original and data obtained by reading the hundredth original, which causes differences between pieces of data obtained by reading the originals.
Up to now, as disclosed in Japanese Patent Application Laid-Open No. 2007-081968, there is known a technology in which, in the case where the flow reading is continuously performed, for each interval between the originals, an average value of light amounts is acquired based on image data corresponding to a predetermined number of pixels at several points in the main scanning direction of a reference target object, and fluctuation amounts of the light amounts are calculated, to thereby correct light distribution fluctuations of illumination in the main scanning direction. In light distribution fluctuation correction, first, before reading the image of the first original among multiple originals, a first white reference member is read by the image reading unit to create shading correction data. Further, before reading the first original, a second white reference member is illuminated by the image reading unit, and light amount values from the illuminated second white reference member are sampled to create first pieces of data.
Subsequently, after reading the first image, the light amount values from the second white reference member illuminated by the image reading unit are sampled to create second pieces of data. The fluctuation amounts of the light amounts are obtained by comparing the first pieces of data with the second pieces of data, and the shading correction data is corrected by using the fluctuation amounts of the light amounts to correct the image data. In the original reading position, the image reading unit reads the image of the original while the original is passing between glass and an opposing member disposed so as to be opposed to the glass. The opposing member opposed to the glass is used as the second white reference member, and reflected light from the opposing member is sampled. Accordingly, without moving the image reading unit from a flow reading position, it is possible to appropriately correct the image data based on fluctuations in the light amounts of the light source.
In order to perform appropriate correction by using the conventional image reading apparatus, the second white reference member (opposing member) read by the image reading unit needs to have uniform reflection property. In a case where light that is reflected by the opposing member and enters a light-receiving unit is specularly reflected light, the light distribution fluctuation correction is performed for a region from which the specularly reflected light enters the light-receiving unit to correct a luminance level of the image to become lower. This may cause such image degradation that the image becomes partially dark.